This invention relates to a fuel assembly for a nuclear reactor, and more particularly to a wire spacer fuel assembly for a fast breeder reactor (FBR).
FIG. 1 shows the cross section of a fuel assembly as taken at a certain position in the axial direction thereof. Regarding the coolant temperature within the fuel assembly, reference is made to FIG. 2 which shows the radial distribution of the coolant temperature in an AB-plane in FIG. 1. A peripheral coolant flow channel 102 contacting with a wrapper tube wall 101 faces a fuel pin, being a heating element, 103 on only one side thereof, so that the temperature of coolant flowing therethrough is lower than the temperature of coolant flowing through an inner coolant flow channel 104. This temperature difference increases as the outlet of the coolant flow channel is nearer, and it reaches about 50.degree. C. at the outlet. Therefore, the temperature of the cladding tube of especially the outermost fuel pin exhibits a difference between the wrapper tube side and the side opposite thereto, and both these sides undergo different expansions in the axial direction due to heat and neutron irradiation. As the result, as indicated by broken lines in FIG. 3, the fuel pin 103 which is fixed at the lowermost end only deforms in a bowing deformation towards the wrapper tube wall 101. The clearance between the outermost fuel pin and the wrapper tube wall is to the extent of 1-2 mm with the diameter of a wire spacer included. When, under this condition, the outermost fuel pin is permitted to freely bow, it is subjected to such extent of bending moment that the displacement of the fuel pin amounts to 10 cm at the top thereof. In consequence, the outermost fuel pin is pushed back by the wrapper tube wall and is deformed as indicated by solid lines in FIG. 3. In the illustration, the interaction of the outermost fuel pin with the inner fuel pin is not taken into consideration, and actually the deformations of the fuel pins become more complicated under interaction among the fuel pins. In case where the fuel pins within the fuel assembly are deformed in this manner, the maximum load acts on the top of the outermost fuel pin, and there is the possibility that the external force exerted from the wrapper tube wall will reach above 1 Kg.
It is a factor for the damage of the fuel pin that the external force acts on the fuel pin to deform the section thereof. Moreover, as shown in FIG. 4, the fuel pin 103 and the wrapper tube wall 101 contact without the intervention of a wire spacer 105 on one side of the top of the outermost fuel pin. It is therefore possible that fretting corrosion will take place in this portion and that the wall thickness of the cladding tube will decrease. Furthermore, when the state of contact with the wrapper tube wall differs on both the sides of the upper parts of the fuel pins in this manner, the fuel pin bundle leans onto one side by the diameter of the wire spacer. As the result, the peripheral coolant flow channel facing the wrapper tube wall on the other side becomes extensive, the temperature difference between the coolant flowing through the peripheral flow channel and that flowing through the inner flow channel increases, and the outermost fuel pin is bowed more.